2005-11-11 Chaoqun Wu, Fudan University 1 Epigenetics — Chromatin based gene control Chaoqun Wu School of Life Sciences, Fudan University 2005-11-11 Chaoqun Wu, Fudan University 2 Genomes and Gene Number 27,000 13,500 30,000 6000 19,000 Gene number 2005-11-11 Chaoqun Wu, Fudan University 3 Muscle cells Nerve cells But if in the same organism, should have the same DNA per cell Different cells have different clusters of genes active Development and differentiation of embryos is controlled by signaling molecules that alter gene expression in cells. 2005-11-11 Chaoqun Wu, Fudan University 4 2005-11-11 Chaoqun Wu, Fudan University 5 Genome is packaged into chromatin 2005-11-11 Chaoqun Wu, Fudan University 6 Basic unit is nucleosome: Consists of a core octamer of histones H2A, H2B, H3 and H4 (2X). Approximately 150 bp DNA wrapped 2X around outside of nucleosome 2005-11-11 Chaoqun Wu, Fudan University 7 Summary: the Regulation of Gene Expression in Eukaryotes 2005-11-11 Chaoqun Wu, Fudan University 8 Transcription and Translation 2005-11-11 Chaoqun Wu, Fudan University 9 Cell-specific regulation of gene expression TRANSCRIPTION Enhancer Promoter Promoter-proximal element Enhancer A specific signal molecule (such as one with the message, “become a muscle cell”) leads to production of regulatory proteins. These proteins bind to regulatory sites in DNA, triggering transcription of cell-specific proteins. Exon Exon ExonIntron Intron 2005-11-11 Chaoqun Wu, Fudan University 10 TFIIB RNAPII TFIIA TFIID TFIIH TFIIF Mediator TFIIE DNA-binding Regulators Transcription involves 4 types of proteins ? DNA-binding regulators ? Chromatin regulators ? Coactivators and corepressors ? Transcription apparatus Chromatin regulators Transcription apparatus Coactivator Gene 2005-11-11 Chaoqun Wu, Fudan University 11 2005-11-11 Chaoqun Wu, Fudan University 12 All eukaryotic genetic information Histone code Genetic code Genomic DNA sequences histone amino-terminal modifications 2005-11-11 Chaoqun Wu, Fudan University 13 Chromatin, the physiological template of all eukaryotic genetic information, is subject to a diverse array of posttranslational modifications that largely impinge on histone amino termini, thereby regulating access to the underlying DNA. Distinct histone amino-terminal modifications can generate synergistic or antagonistic interaction affinities for chromatin- associated proteins, which in turn dictate dynamic transitions between transcriptionally active or transcriptionally silent chromatin states. The combinatorial nature of histone amino-terminal modifications thus reveals a "histone code" that considerably extends the information potential of the genetic code. Thomas Jenuwein and C. David Allis (2001), Science, 293:1074-1080 , 2005-11-11 Chaoqun Wu, Fudan University 14 ? code Genetic code Histone code Genomic DNA sequences histone amino-terminal modifications ? 2005-11-11 Chaoqun Wu, Fudan University 15 ?Introduction ?Chromatin has important role in regulation of gene expression ?DNA methylation ?Histone modifications ?Chromatin remodeling ? Rnomics ?Epigenetics and Diseases ?Summary 2005-11-11 Chaoqun Wu, Fudan University 16 Part I. Introduction 2005-11-11 Chaoqun Wu, Fudan University 17 随着为期 随着为期 10年耗资 年耗资 30亿美元的 亿美元的 HGP宣告结束，人们迈 宣告结束，人们迈 进后基因组时代。 进后基因组时代。 通过大量的生物信息学分析，发 通过大量的生物信息学分析，发 现相当比重的疾病患者基因组水平上并没有发生突 现相当比重的疾病患者基因组水平上并没有发生突 变，而基因组中的调控序列在基因选择性表达中的 变，而基因组中的调控序列在基因选择性表达中的 具有重要生物学意义。阐明基因选择性表达所依赖 具有重要生物学意义。阐明基因选择性表达所依赖 的调控信息及其相互作用的分子机制，是揭示生命 的调控信息及其相互作用的分子机制，是揭示生命 现象本质的核心问题，是结构基因组之后功能基因 现象本质的核心问题，是结构基因组之后功能基因 组研究的重要内容，因此有人提出表观遗传学，并 组研究的重要内容，因此有人提出表观遗传学，并 日益成为遗传学研究的一个新的热点。 日益成为遗传学研究的一个新的热点。 2005-11-11 Chaoqun Wu, Fudan University 18 Approximately 27,000 genes in the human genome. Roughly 10,000 genes expressed In individual cells. A single somatic cell nucleus is sufficient to generate an organism. Complexity in form and function generated by differential gene expression. 2005-11-11 Chaoqun Wu, Fudan University 19 Same genome, different epigenome ● Variability in CpG methylation at the agouti locus causes differences in coat color among genetically identical mice. ● Maternal nutrition affects the phenotype of offspring by influencing the degree of CpG methylation at the agouti locus. (Molec Cell Biol, Aug 2003) 2005-11-11 Chaoqun Wu, Fudan University 20 Epigenetic modifications are changes in the chromatin structure which result either in: ? the activation of the genome without modifying (mutating) the primary nucleotide sequence ? repression of defined regions of the genome without modifying (mutating) the primary nucleotide sequence 2005-11-11 Chaoqun Wu, Fudan University 21 The main topics of Epigenetics are involved in transcriptional regulation in the context of chromatin. 2005-11-11 Chaoqun Wu, Fudan University 22 2005-11-11 Chaoqun Wu, Fudan University 23 Terms of Epigenetics: 1. The study of heritable changes in gene function that occur without a change in the sequence of the DNA. 2. The study of certain processes that occur in embryonic development. 2005-11-11 Chaoqun Wu, Fudan University 24 1. The study of heritable changes in gene function that occur without a change in the sequence of the DNA. ?The process of methylation (1). In chemistry, the addition of a methyl group to a molecule. (2). On a protein level, the addition of a methyl group or groups to the amino acid arginine or lysine in a protein. (3). On a DNA level, the addition of a methyl group to a cytosine residue to convert it to 5-methyl- cytosine. 2005-11-11 Chaoqun Wu, Fudan University 25 Methylation of DNA occurs at CpG sites, where cytosine (C) lies next to guanine (G). The CpG sites are in regions near the promoters of a genes. These regions are known as CpG islands. The state of methylation of CpG islands is critical to both gene activity and gene expression. 2005-11-11 Chaoqun Wu, Fudan University 26 ?The process of chromatin remodeling. Dynamic structural changes to the chromatin occurring throughout the cell division cycle and other activities. These changes range from the local changes necessary for transcriptional regulation to global changes necessary for chromosome segregation. Chromatin remodeling is an epigenetic phenomenon. 2005-11-11 Chaoqun Wu, Fudan University 27 2. The study of certain processes that occur in embryonic development. ?X inactivation (the inactivation of one X chromosome in females) X inactivation: The phenomenon in a female by which one X chromosome (either the maternally or paternally derived X) is randomly (by chance) inactivated in an early embryonic cell, with fixed inactivation of that same X in all cells descended from that cell. 2005-11-11 Chaoqun Wu, Fudan University 28 The change that occurs with X inactivation is epigenetic: it is a heritable change in gene function without a change in the sequence of the DNA. X inactivation is not restricted to females. It also occurs in males with Klinefelter syndrome who have more than one X chromosome. The phenomenon of X inactivation is also called lyonization after the English geneticist Mary Lyon (1925-) who first described it. 2005-11-11 Chaoqun Wu, Fudan University 29 ?The phenomenon of gene silencing. Gene silencing: A mechanism by which cells shut down large sections of chromosomal DNA. Gene silencing is done by incorporating the DNA to be silenced into a form of DNA called heterochromatin that is already silent. The process of gene silencing is important for the differentiation of many different types of cells. 2005-11-11 Chaoqun Wu, Fudan University 30 Epigenetics versus Cytomics 基 因 组 学 结构基因组学 Structure Genetics 功能基因组学 Functional Genetics 蛋白质组学 Proteomics 表观遗传学 Epigenetics 细胞形态学 Cytomophorlogy 细胞生理学 Cell Physiology 细 胞 生 物 学 细胞病理学 Cellular Pathology 细胞组学 2005-11-11 Chaoqun Wu, Fudan University 31 Epigenetics versus developmental Biology 基 因 组 学 结构基因组学 Structure Genetics 功能基因组学 Functional Genetics 蛋白质组学 Proteomics 表观遗传学 Epigenetics 个体形态 发生学 Morphogenesis 胚胎学 Embreology 生 物 学 病理学 Pathology 发育生物学 2005-11-11 Chaoqun Wu, Fudan University 32 Epigenetical regulation Based on –Reversible changes of the DNA –Modifications of the chromatin –Modifications of post- transcriptional processes 2005-11-11 Chaoqun Wu, Fudan University 33 Genetics versus Epigenetics Genetics 经典模式 致癌剂DNA突变 Epigenetics模式 基因序列没有改变，而是通过碱基、 组蛋白修饰、区域化等调控基因的表 达，从而引起一系列的表型效应。 2005-11-11 Chaoqun Wu, Fudan University 34 There are several features that distinguish epigenetics from conventional genetic mechanisms: ?Reversibility; ?Position effects, i.e., the ability to act over unexpected distances larger than a single gene; ?Apparent mutations at unexpectedly high frequency; ?Involvement of gene domains. 2005-11-11 Chaoqun Wu, Fudan University 35 The epigenome is reprogrammed during development 2005-11-11 Chaoqun Wu, Fudan University 36 Dynamic reprogramming of the epigenome during development. Epigenetic marks, including DNA methylation and genomic imprinting, are reprogrammed during normal gametogenesis. Primordial germ cells undergo epigenetic erasure as they migrate along the genital ridge, and epigenetic marks are reestablished during gametogenesis, differentially in sperm (blue) and egg (pink). For example, after fertilization, there is active demethylation of the paternal pronucleus, and then a second wave of passive demethylation of the zygote genome. Imprinted genes (dotted line) are protected from this erasure. During development, tissue-specific epigenetic patterns emerge. The drawing is stylized, as details are unknown. (Am J Hum Genet, 74:599-609 2004) 2005-11-11 Chaoqun Wu, Fudan University 37 I. Definition Epigenetics is defined as modifications of the genome, heritable during cell division, that do not involve a change in the DNA sequence. The epigenome is the overall epigenetic state of an organism. 2005-11-11 Chaoqun Wu, Fudan University 38 Epigenetics – definition Any changes in gene expression resulting from either a DNA and chromatin modification or resulting from a post-transcriptional mechanism However, it does not reflect a difference in the DNA code 2005-11-11 Chaoqun Wu, Fudan University 39 什么是表观遗传学 (Epigenetics)？ 所谓表观遗传就是不基于 所谓表观遗传就是不基于 DNA差异的核酸遗 差异的核酸遗 传。即细胞分裂过程中 传。即细胞分裂过程中 , DNA序列不变的前 序列不变的前 提下 提下 , 全基因组的基因表达调控所决定的表 全基因组的基因表达调控所决定的表 型遗传 型遗传 ,涉及染色质重编程（ 涉及染色质重编程（ Remodeling)、 、 整体的基因表达调控（如隔离子，增强子， 整体的基因表达调控（如隔离子，增强子， 弱化子， 弱化子， DNA甲基化，组蛋白修饰等功能 甲基化，组蛋白修饰等功能 ), 及基因型对表型的决定作用。 及基因型对表型的决定作用。 2005-11-11 Chaoqun Wu, Fudan University 40 In biology today, epigenetics has two closely related meanings. 1. The study of the processes involved in the unfolding development of an organism. This includes phenomena such as X- chromosome inactivation in mammalian females, and gene silencing within an organism. 2005-11-11 Chaoqun Wu, Fudan University 41 2. The study of heritable changes in gene function that occur without a change in the sequence of nuclear DNA. This includes the study of how environmental factors affecting a parent can result in changes in the way genes are expressed in the offspring. 2005-11-11 Chaoqun Wu, Fudan University 42 In both cases mentioned above, the object of study includes How gene regulatory information that is not expressed in DNA sequences is transmitted from one generation (of cells or organisms) to the next. 2005-11-11 Chaoqun Wu, Fudan University 43 II. Examples of epigenetic phenomena 1. Paramutation(副突变 副突变 ) 2. Gene silencing by DNA methylation 3. Parental (genomic) Imprinting 4. Position effect variegation(位子效应多样性） 位子效应多样性） 5. X-chromosome dose compensation 6. RNA interference (RNAi) 7. Transvection(等位反式效应 等位反式效应 ) 2005-11-11 Chaoqun Wu, Fudan University 44 1. Paramutation effects Heritable change in gene expression induced by allele (等位基因 等位基因 ) interactions Roles are – localizing recombination to low-copy sequences within the genome – establishing and maintaining chromatin domain boundaries – and providing a mechanism for plants to transmit an environmentally influenced expression state to progeny 2005-11-11 Chaoqun Wu, Fudan University 45 Paramutation in maize Brink (1956) observed in maize heritable changes of rl alleles in heterozygous F1 (rl1 rl2): allelic interactions give new phenotype Later also described for other genes and in other species Also transgenes and transposable elements can display paramutation-like phenomena Is heritable reduction in expression of one of the alleles 2005-11-11 Chaoqun Wu, Fudan University 46 A series of crosses depicting paramutation. The B-I mutation produces pigmented plants, whereas the B mutation produces nearly unpigmented plants. Normally, when B-I is crossed with recessive colorless alleles of the b gene, the resulting plants are pigmented. However, when B-I and B plants are intercrossed, the F1 plants are essentially unpigmented, like the B homozygotes. Thus, B-I is altered by being in the same genome as B , indicated by the B-I* designation. If this outcome were due simply to the dominance of B to B-I, then a self-cross of the F 1 plants should generate B-I colored homozygotes as approximately 1/4 of the F 2 progeny. Instead, no F 2 are pigmented. Intercrosses of the F 2 and of further generations do not restore the pigmented phenotype. Thus, B-I is said to have been paramutated by virtue of being in the same nucleus with the B allele. 2005-11-11 Chaoqun Wu, Fudan University 47 2. DNA silencing by DNA methylation Plants and filamentous fungi share with mammals enzymes responsible for DNA methylation. In these organisms, DNA methylation is associated with gene silencing and transposon control. However, plants and fungi differ from mammals in the genomic distribution, sequence specificity, and heritability of methylation. Transposons play a role in establishing methylation patterns and the epigenetic consequences of their perturbation Martienssen, Science 2001 2005-11-11 Chaoqun Wu, Fudan University 48 Properties DNA methylation CpG repeats may serve as trigger to induce DNA methylation and so silence gene expression, but exceptions exist. DNA methylation especially in plants and animals DNA methylation and histone acetylation are antagonistic mechanisms in chromatin modulation 2005-11-11 Chaoqun Wu, Fudan University 49 3. Parental (genomic) Imprinting Imprinting can be loosely defined as the gamete-of-origin dependent modification of phenotype Imprinting results from changes in gene expression rather than aberrant transmission of alleles “No matter how loving a couple may seem, their genes are waging a molecular battle in the developing embryo: his genes do what they can to promote their own propagation, while her genes fight back to make sure they are not overrun” 2005-11-11 Chaoqun Wu, Fudan University 50 Imprinting mechanism pigment gene is paternally imprinted pigment gene is maternally imprinted 2005-11-11 Chaoqun Wu, Fudan University 51 Two examples of a hypothetical imprinted gene responsible for body color. (LEFT) In this example the (maternal allele is inactivated). Matings between a male who possesses the allele for pigment and a female who possesses the allele for no pigment produces offspring that show only the pigmented phenotype. In this example, the mother's allele is imprinted and inactivated in the offspring. Therefore, the only actively-expressing allele is the father's pigment allele, which is not imprinted in the offspring. (RIGHT) In this example the pigment gene is paternally imprinted (paternal allele is inactivated). Matings between a male who possesses the allele for pigment and a female who possesses the allele for no pigment produces offspring that show only the pigmented phenotype. In this example, the father's allele is imprinted and inactivated in the offspring. Therefore, the only actively expressing allele is the mother's no pigment allele, which is not imprinted in the offspring. 2005-11-11 Chaoqun Wu, Fudan University 52 4. Position effect variegation(PEV) In Drosophila melanogaster discovered PEV results from chromosome rearrangements with breakpoints in both the euchromatin and heterochromatin observed as the mosaic inactivation of genes near the novel junctions. All euchromatic genes examined thus far have been shown to be subject to PEV, indicating the universal nature of this effect. 2005-11-11 Chaoqun Wu, Fudan University 53 PEV – scheme A euchromatic gene placed in the vicinity of hetero-chromatin by a chromosomal rearrangement generally exhibits position effect variegation (PEV), a clonally inherited pattern showing gene expression in some somatic cells but not in others. Chromocemter 2005-11-11 Chaoqun Wu, Fudan University 54 5. X – dose compensation Drosophila XX – XY sex system Males: 1 X chromosome Females: 2 X chromosomes Compensation for equal expression: Male X chromosome transcription increased 2-fold by Male Specific Lethal (MSL) complex 2005-11-11 Chaoqun Wu, Fudan University 55 Male Specific Lethal complex MSL = 5 proteins and 2 non-coding RNAs If mutated, dosage compensation fails and males die Binds only to the male X chromosome Spreads active chromatin from about 35 entry sites on male X 2005-11-11 Chaoqun Wu, Fudan University 56 Model for MSL binding to chromatin 2005-11-11 Chaoqun Wu, Fudan University 57 MSL binding on X chromosome 2005-11-11 Chaoqun Wu, Fudan University 58 6. RNA interference (RNAi) RNAi (RNA interference) refers to the introduction of homologous double stranded RNA (dsRNA) to specifically target a gene's product, resulting in null (recessive) phenotypes. So, it is the experimental silencing of genes by the introduction of double- stranded RNA 2005-11-11 Chaoqun Wu, Fudan University 59 How RNAi works RNAi medias post-transcriptional silencing 2005-11-11 Chaoqun Wu, Fudan University 60 RNAi-mediated Epigenetic Control of the Genome A model for siRNA-dependent initiation of heterochromatin assembly by RITS. The RITS complex is programmed by Dcr1- produced siRNAs to target specific chromosome regions by sequence-specific interactions involving either siRNA-DNA or siRNA-nascent transcript (blue arrows) base pairing. Nuc, nucleosome; red triangle, K9-methylation on the amino terminus of histone H3. Science 303:672-676, 2004 2005-11-11 Chaoqun Wu, Fudan University 61 RITS (RNA-induced initiator of transcriptional gene silencing) Mechanism for RNAi-mediated targeting of heterochromatin. siRNAs may serve as primers for RdRp to produce additional dsRNAs from single stranded transcripts. siRNAs join the RITS complex to provide specificity for localization of the complex to homologous sequences. The recruitment of histone modifying activities, such as the Clr4 H3 Lys9 HMT, by the RITS complex creates a ‘histone code’ for the binding of the chromodomaincontaining proteins Chp1 and Swi6/HP1. The binding of Chp1 serves to stabilize RITS to the loci while the binding of Swi6/HP1 results in heterochromatic spreading as a result of the combined activity of the H3 Lys9 methylation and associated Swi6/HP1. Current Opinion in Cell Biology 2004, 16:230–238 2005-11-11 Chaoqun Wu, Fudan University 62 7. Transvection effects Transvection is a process by which the activity of a gene is altered when it is paired with a homologue (in trans) Example is the intragenic complementation seen at the yellow locus of Drosophila. Here, transvection can occur by enhancers of one allele acting in trans on the promoter of a paired homologue Second mechanism for transvection arise from changes in gene structure caused by pairing of two structurally dissimilar alleles 2005-11-11 Chaoqun Wu, Fudan University 63 Transvection is epigenetic modification which is induced in one allele when the other one is itself modified Transvection effects leading to modifications of gene expression and regulation were subsequently demonstrated in a number of Drosophila genes (for review see Pirrotta, 1999) and in the fungus Neurospora crassa (Aramayo and Metzenberg, 1996). 2005-11-11 Chaoqun Wu, Fudan University 64 Transvection at yellow. In the y 2 mutation, the insertion of the gypsy element between the upstream enhancers and the promoter acts as an insulator to block the action of the enhancers on the promoter. A homologously paired yellow gene that lacks its own enhancers, promoter, and part of the coding region can still provide enhancer action by inducing the y 2 gene to fold, looping out the gypsy enhancer and bringing the y 2 enhancers close to the promoter. 2005-11-11 Chaoqun Wu, Fudan University 65 Transvection is thought to involve both modulations in enhancer-promoter interactions as well as possible changes in chromosome topology surrounding a chromatin insulator. Transvection is found at many loci in flies (Pirrotta, 1999), and similar effects have been reported in fungi (Aramayo and Metzenberg, 1996), plants (Matzke et al., 2001), and human tissue culture cells (Ashe et al., 1997). It seems that transvection may exemplify a homology-dependent mechanism of gene regulation that has been conserved during evolution; therefore, knowledge gained through studies of transvection should inform us about the more general functions of regulatory elements. 2005-11-11 Chaoqun Wu, Fudan University 66 III. Chromatin structure What is chromatin ? Chromatin – dynamic condensed form during M-phase of mitosis amorphous long fibers in the nucleus – DNA (Chromosomes) – Protein (Histones,non-histone proteins) – RNA (small amounts) ~ equal amounts 2005-11-11 Chaoqun Wu, Fudan University 67 In Eucaryotes, DNA is Enclosed in a Cell Nucleus 2005-11-11 Chaoqun Wu, Fudan University 68 nucleosome Chromatin: the physiological template of our genome from Horn and Peterson Science, 2002 2005-11-11 Chaoqun Wu, Fudan University 69 Basic chromatin elements Structural elements ? Histones ? DNA Cis elements ? Methylated DNA ? Demethylated DNA ? Promoters ? Enhancers ? Locus control regions ? Matrix-attachment sites Trans elements ? Transcription factors ? RNA polymerases ? Chromatin-remodelling complexes ? Histone-modification enzymes 2005-11-11 Chaoqun Wu, Fudan University 70 Packaged into a Set of Chromosomes 2005-11-11 Chaoqun Wu, Fudan University 71 DNA Packaging Why package DNA? –Length of DNA = 2 meters, Size of nucleus = 5 μm –Need to separate strands during cell division –Need to access for Transcription 2005-11-11 Chaoqun Wu, Fudan University 72 Levels of Packaging 1. Nucleosome 2. 30 nm fiber/Solenoid 3. Loops 4. Rossettes 5. Chromosome 2005-11-11 Chaoqun Wu, Fudan University 73 “beads on a string” 11 nm 30 nm 30 nm fiber 300 nm 700 nm Metaphase chromosome 1400 nm Alberts et. al. Molecular Biology of the Cell 2005-11-11 Chaoqun Wu, Fudan University 74 Higher order chromatin structure Euchromatin – less condensed – nuclease sensitive – less methylated – actively transcribed genes Heterochromatin – highly condensed – nuclease insensitive – highly methylated “silenced” genes Phosphorylation Acetylation Methylation Ubiquitination 2005-11-11 Chaoqun Wu, Fudan University 75 Chromatin Repressed chromatin Active chromatin 2005-11-11 Chaoqun Wu, Fudan University 76 Histones Histone Mol. Weight No. of amino acids % basic residues (Arg + Lys) H1 21,130 223 30.8 H2A 13,960 129 20.2 H2B 13,774 125 22.4 H3 15,273 135 22.9 H4 11,236 102 24.5 Highly conserved In Nucleosome Histone octamer: 2 x H2a, 2 x H2b, 2 x H3 and 2 x H4 2005-11-11 Chaoqun Wu, Fudan University 77 Histones Gene Expression – May be co-regulated – Gene-specific element (red) “GRGCGCAGATTTCGG” found in upstream region of Giardia histones Histone post-translational modification (1) phosphorylation, (2) methylation, (3) acylation, (4) ADP- ribosylation and (5) ubiquination affects net charge, shape and function H2a H2b H3 H4 2005-11-11 Chaoqun Wu, Fudan University 78 The Nucleosome Modified from Figure 8-9, Page 252 from: Essential Cell Biology by Alberts et al. 1997, Garland Publishing Inc. New York, NY 2005-11-11 Chaoqun Wu, Fudan University 79 Nucleosome to Solenoid (螺线管) Histone H1 Nucleosome 30 nm Fiber/ Solenoid Modified from Figure 8-10, Page 253 from: Essential Cell Biology by Alberts et al. 1997, Garland Publishing Inc. New York, NY Histone H1 2005-11-11 Chaoqun Wu, Fudan University 80 The global structure of chromosomes Chromatin is arranged in domains – Large loops of DNA attached to common scaffold Heterochromatin is compact and inaccessible for transcription Chromatin structure is important for the mitosis, recombination and global control of gene expression 2005-11-11 Chaoqun Wu, Fudan University 81 Lampbrush Chromosomes (灯刷染体 ) contain loops of decondensed chromatin 2005-11-11 Chaoqun Wu, Fudan University 82 Chromosomes Exist in Different States Throughout the Life of a Cell 2005-11-11 Chaoqun Wu, Fudan University 83 Chromatin characteristics Closed,silent chromatin (heterochromatin) is likely to: ? be DNase-I insensitive ? contain hypermethylated DNA ? contain hypoacetylated histones ? contain histone-3 that is methylated at lysine-9 ? be associated with repressive chromatin-remodelling complexes Open,expressing chromatin (euchromatin) is likely to: ? be DNase-I sensitive ? contain hypomethylated DNA ? contain hyperacetylated histones ? contain histone-3 that is demethylated at lysine-9 ? be associated with activating chromatin-remodelling complexes